Solar cell, multi-junction solar cell, solar cell module, and solar power generation system

ABSTRACT

According to one embodiment, a solar cell includes a first electrode, a second electrode, and a photoelectric conversion layer disposed between the first electrode and the second electrode. When a transmittance of the solar cell is measured in a wavelength range of 700 to 1000 nm, an average of the transmittance of the solar cell is 60% or more.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Applications No. 2018-175275, filed on Sep. 19, 2018,and No. 2019-033074, filed on Feb. 26, 2019; the entire contents ofwhich are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a solar cell, amulti-junction solar cell, a solar cell module, and a solar powergeneration system.

BACKGROUND

A multi-junction (tandem) solar cell is a solar cell having a higherefficiency. The tandem solar cell can use a cell with high spectralsensitivity for each wavelength band, and therefore can have a higherefficiency than a single junction battery. In addition, as a top cell ofthe tandem solar cell, a coprous oxide compound made of an inexpensivematerial and having a wide band gap has been expected. However, anefficiency of about 8% has been reported for a coprous oxide solar cellmanufactured by oxidizing a copper foil, but this value is lower than atheoretical critical efficiency. A reason for this is considered to beas follows. That is, a heterogeneous phase such as copper oxide on anoutermost surface is removed by etching after a copper foil is oxidized,but the heterogeneous phase cannot be completely removed, and an elementconstituting an etching solution remains. Therefore, a favorable pnjunction cannot be formed. In this method, after a foil having athickness of about 0.1 mm is oxidized, the foil needs to be polished soas to have a thickness of about 20 μm. This makes it difficult toincrease the area.

Meanwhile, there is an example in which a thin film is formed by amethod using a reaction in a liquid phase or the like, but an efficiencyis about 4% at a maximum. A main cause for this is considered to bethat, not only a heterogeneous phase but also impurities contained in asolution are incorporated into a film to serve as a center ofrecombination of photoexcited carriers. Such a thin film cannot be usedfor a top cell of the tandem solar cell because of absorbing also lighthaving a wavelength of 600 nm or more which is not originally absorbedby a thin film. In general, a sputtering method is well known as amethod for manufacturing a thin film with a small amount of contaminatedimpurities, and there is a reported example in which a thin film hasbeen manufactured by this method. However, a conversion efficiency is 1%or less. A reason for this is considered to be that, a heterogeneousphase of copper or copper oxide is easily generated even withoutcontamination of impurities, and pure coprous oxide is not easilyobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual cross-sectional view of a solar cell according toa first embodiment.

FIG. 2 is a schematic diagram of a method for measuring a photoelectricconversion layer using X-ray photoelectron spectroscopy.

FIG. 3 is a flowchart of a method for manufacturing the solar cellaccording to the first embodiment.

FIG. 4 is a conceptual cross-sectional view of a multi-junction solarcell according to a second embodiment.

FIG. 5 is a conceptual diagram of a solar cell module according to athird embodiment.

FIG. 6 is a conceptual cross-sectional view of the solar cell moduleaccording to the third embodiment.

FIG. 7 is a conceptual diagram of a solar power generation systemaccording to a fourth embodiment.

DETAILED DESCRIPTION

Hereinafter, preferred embodiments will be described in detail withreference to the accompanying drawings.

According to one embodiment, a solar cell includes a first electrode, asecond electrode, and a photoelectric conversion layer disposed betweenthe first electrode and the second electrode. When a transmittance ofthe solar cell is measured in a wavelength range of 700 to 1000 nm, anaverage of the transmittance of the solar cell is 60% or more.

First Embodiment

A first embodiment relates to a solar cell. FIG. 1 shows a conceptualdiagram of a solar cell 100 according to the first embodiment. As shownin FIG. 1, the solar cell 100 according to the first embodiment includesa substrate 1, a first electrode 2 on the substrate 1, a photoelectricconversion layer 3 on the first electrode 2, an n-type layer 4 on thephotoelectric conversion layer 3, and a second electrode 5 on the n-typelayer 4. An intermediate layer (not shown) may be included between thefirst electrode 2 and the photoelectric conversion layer 3 or betweenthe n-type layer 4 and the second electrode 5.

(Substrate)

It is desirable to use white sheet glass as the substrate 1 according tothe embodiment. It is also possible to use general glass such as quartz,soda lime glass, or chemically tempered glass, a metal plate such asstainless steel (SUS), W, Ta, Al, Ti, or Cr, or a resin such aspolyimide or an acrylic resin.

(First Electrode)

The first electrode 2 according to the embodiment is a layer existingbetween the substrate 1 and the photoelectric conversion layer 3. InFIG. 1, the first electrode 2 is in direct contact with the substrate 1and the photoelectric conversion layer 3. As the first electrode 2, atransparent conductive film or a laminate of a metal film, a transparentconductive film, and a metal film is preferable. Examples of thetransparent conductive film include indium tin oxide (ITO), Al-dopedzinc oxide (AZO), boron-doped zinc oxide (BZO), gallium doped zinc oxide(GZO), fluorine-doped tin oxide (FTO), antimony-doped tin oxide (ATO),titanium-doped indium oxide (ITiO), indium zinc oxide (IZO), indiumgallium zinc oxide (IGZO), Ta-doped tin oxide (SnO₂:Ta), Nb-doped tinoxide (SnO₂:Nb), W-doped tin oxide (SnO₂:W), Mo-doped tin oxide(SnO₂:Mo), F-doped tin oxide (SnO₂:F), and hydrogen-doped indium oxide(IOH), and are not particularly limited.

The transparent conductive film may be a laminated film having aplurality of films, and a film such as tin oxide may be included in thelaminated film in addition to the above oxide. Examples of a dopant to afilm such as tin oxide include In, Si, Ge, Ti, Cu, Sb, Nb, F, Ta, W, Mo,F, and Cl, and are not particularly limited. Examples of the metal filminclude films of Mo, Au, Cu, Ag, Al, Ta, and W, and are not particularlylimited.

The first electrode 2 may be an electrode having a dot-like, line-like,or mesh-like metal disposed on the transparent conductive film. At thistime, the dot-like, line-like, or mesh-like metal is disposed betweenthe transparent conductive film and the photoelectric conversion layer3, or on the transparent conductive film on the opposite side to thephotoelectric conversion layer 3. The dot-like, line-like, or mesh-likemetal preferably has an aperture ratio of 50% or more with respect tothe transparent conductive film. Examples of the dot-like, line-like, ormesh-like metal include Mo, Au, Cu, Ag, Al, Ta, and W, and are notparticularly limited.

For the first electrode 2, a metal substrate may be used in place of thetransparent conductive film. Examples of the metal substrate include W,Cr, Ti, Ta, Al, and SUS (for example, SUS 430). On the metal substrate,the photoelectric conversion layer 3 described later may be directlyformed.

(Photoelectric Conversion Layer)

The photoelectric conversion layer 3 according to the embodiment is ap-type compound semiconductor layer. The photoelectric conversion layer3 is a layer existing between the first electrode 2 and the n-typecompound semiconductor layer 4. Incidentally, hereinafter, the n-typecompound semiconductor layer is referred to as an n-type layer. In FIG.1, the photoelectric conversion layer 3 is in direct contact with thefirst electrode 2 and the n-type layer 4. The photoelectric conversionlayer 3 is a layer containing coprous oxide. Coprous oxide is non-dopedor doped coprous oxide. Coprous oxide typically has a thickness of 500nm or more and 0.3 mm or less, but the thickness is not limited thereto.Since coprous oxide is inexpensive compared to a compound having achalcopyrite structure, the solar cell 100 can reduce cost thereof.Coprous oxide has a band gap of about 2.1 eV and has a wide band gap. Ina case where the photoelectric conversion layer 3 of the solar cell 100according to the present embodiment has a wide band gap, when a solarcell having the narrow band gap photoelectric conversion layer 3 such asSi is used as a bottom cell to form multi-junction, the solar cell 100according to the first embodiment has high transmittance of light havinga wavelength contributing to power generation on a bottom cell side.Therefore, the solar cell 100 according to the first embodiment ispreferable in that the amount of power generation on the bottom cellside is high. In a case where the solar cell 100 according to the firstembodiment is used as a multi-junction solar cell, the solar cell 100according to the first embodiment is preferably disposed on a lightincident side.

Here, a method for manufacturing the photoelectric conversion layer 3included in the solar cell according to the present embodiment will bedescribed.

The photoelectric conversion layer 3 is formed by sputtering. First, bysupplying, for example, RF power from a high frequency power supply to atarget of oxygen-free copper having a purity of 4 N or more in a mixedgas of Ar and oxygen, a thin film of coprous oxide can be obtained.Other power such as a DC power supply may be supplied instead of the RFpower supply. At this time, existence of an oxide on at least a part ofan electrode, for example, a substrate during sputtering, makes itpossible to appropriately suppress a reaction between oxygen and theelectrode (corresponding to the substrate at the time of film formation)at the time of formation of coprous oxide, and makes it easy to obtain afavorable crystal. For example, use of a transparent conductive oxidemakes use thereof as an electrode possible, and therefore is desirablefrom a viewpoint of ease of configuration. Therefore, an oxidepreferably exists on at least a part of the electrode. Power used forsputtering may be direct current. Incidentally, at this time, thesubstrate is heated to 400° C. or higher and 700° C. or lower,preferably 450° C. or higher and 550° C. or lower. In this way, acoprous oxide thin film can be formed. A heterogeneous phase such ascopper or copper oxide may be formed depending on a flow rate ratiobetween Ar and oxygen. In order to suppress formation of copper orcopper oxide, for example, it is necessary to adjust a ratio between anAr flow rate and an oxygen flow rate. Note that copper oxide is CuO.

Since sputtering is film formation under reduced pressure, sputtering isperformed in an environment with almost no oxygen. If a film formingtemperature of the photoelectric conversion layer 3 is high, metal iseasily reduced. In order to manufacture coprous oxide, moderateoxidation is necessary. Therefore, it is necessary to increase anoxidizing agent (i.e., oxygen) in an easily reducible environment havinghigh temperature. In other words, because an environment is under aneasily reducible atmosphere with high temperature, it is necessary toincrease an oxygen flow rate. When the photoelectric conversion layer 3is formed at a high temperature, also by increasing an oxygen flow rate,it is possible to obtain the photoelectric conversion layer 3 havinghigh quality.

For a substrate on which coprous oxide is to be formed, a transparentcomponent is preferably used. This is because, by using a transparentcomponent, it is possible to effectively use light in a wavelength rangewhich cannot be absorbed by coprous oxide on the opposite side to alight incident direction utilizing a fact that the component has a widegap.

Copper or copper oxide becomes a heterogeneous phase in thephotoelectric conversion layer 3 included in the solar cell according tothe present embodiment. Therefore, by manufacturing a solar cell inwhich the number of these heterogeneous phases are reduced, theefficiency of the solar cell can be improved.

A heterogeneous phase included in the photoelectric conversion layer 3largely affects a photoelectric conversion efficiency (also referred toas a conversion efficiency) of the solar cell. As described above,existence of a heterogeneous phase may serve as a recombination centerof photoexcited carriers to reduce conversion efficiency, or may reducethe quality of the photoelectric conversion layer 3 itself. This isbecause copper-derived leakage or the like occurs to deteriorate a fillfactor (FF). Furthermore, this is because promotion of recombination orthe like adversely affects the entire cell characteristic in a casewhere copper oxide is contained. In addition, absorption of light in anunintended wavelength range leads to reduction in conversion efficiencyof a bottom cell in a case where a multi-junction solar cell describedlater is formed.

In order to measure a heterogeneous phase existing in the photoelectricconversion layer 3 of the solar cell, analysis is performed by an X-raydiffraction method. Even in a case where no heterogeneous phase isdetected in the photoelectric conversion layer 3, a high conversionefficiency is not necessarily obtained. This is because a heterogeneousphase which cannot be measured by the X-ray diffraction method exists.Therefore, by measuring the transmittance of the solar cell, it ispossible to accurately measure existence of a heterogeneous phase in thephotoelectric conversion layer 3. Measurement of the transmittance is ananalytical method for measuring the ratio of light which has passedthrough a solar cell by irradiating the solar cell with light havingvarious wavelengths. The measuring method will be described later indetail. As the number of heterogeneous phases (included in thephotoelectric conversion layer 3) decreases, the transmittance of thesolar cell increases, and the efficiency of the solar cell can beimproved. Therefore, an average transmittance of the solar cell in awavelength range of 700 to 1000 nm is preferably 60% or more. This isbecause a high conversion efficiency can be achieved by the averagetransmittance in this range. In a case of a multi-junction solar celldescribed later, a bottom cell can efficiently absorb light because atop cell transmits light in a wavelength range of 700 to 1000 nm well.In addition, the transmittance is more preferably 70% or more. This isbecause a higher conversion efficiency can be achieved by the averagetransmittance in this range. If the average transmittance in awavelength range of 700 to 1000 nm is less than 60%, the amount of powergeneration in a bottom cell decreases, and the total amount of powergeneration decreases. Therefore, this is not preferable.

Here, a method for measuring a transmittance will be described. Formeasurement of a transmittance, ultraviolet-visible near-infraredspectroscopy is used. As an apparatus, an ultraviolet-visiblenear-infrared spectrophotometer, for example, a model: UV-3101PCmanufactured by Shimadzu Corporation is used. A sample is placed justbefore an integrating sphere, and a transmittance in the targetwavelength range of 700 to 1000 nm is measured. For measurement of atransmittance, a solar cell is divided into four equal parts in each oflongitudinal and lateral directions. Next, by excluding a scribedportion, five points in a portion where the photoelectric conversionlayer 3 exists are selected as measurement points, and a transmittanceat each of the points is measured. The transmittance at each of themeasurement points is obtained by determining a transmittance at every 1nm in a range of 700 to 1000 nm and by averaging the determinedtransmittances. Thereafter, by averaging the transmittances at thesefive measurement points, an average transmittance of a solar cell to bemeasured can be determined. These measurement points are selected from arange of 5% or less from intersections of lines obtained by equallydividing the solar cell into four equal parts in each of longitudinaland lateral directions with respect to the longitudinal and laterallengths of the solar cell to be measured. Note that a measurement rangeis set within a narrower range than a scribe interval.

In a case where the photoelectric conversion layer 3 is applied not ontoa transparent electrode but onto a metal electrode, it is possible toevaluate a heterogeneous phase existing in the photoelectric conversionlayer 3 by measuring a reflectance. This is because reflected lightdecreases when a heterogeneous phase exists in the photoelectricconversion layer 3 and light in the corresponding wavelength range isabsorbed therein. If surface roughness is large, it is necessary toconsider also reflected light spreading to the surroundings.

The photoelectric conversion layer 3 included in the solar cellaccording to the present embodiment preferably has an averagereflectance of 50% or more in a wavelength range of 700 to 1000 nm. Thisis because, the amount of light in the corresponding wavelength rangeabsorbed in the photoelectric conversion layer 3 is small, and the filmquality is favorable with an average reflectance in this range. Theaverage reflectance is more preferably 58% or more. This is because thefilm quality is better with an average reflectance of 58% or more.

For measurement of a reflectance, the ultraviolet-visible near-infraredspectrophotometer used for the measurement of a transmittance is used. Asample is set such that an incident angle is 5 degrees and a reflectionangle is 5 degrees, and the reflectance of light in the targetwavelength range is evaluated. Also in the method for measuring areflectance, an average reflectance can be determined by setting ameasurement point similarly to the transmittance.

When constituent elements of the photoelectric conversion layer 3included in the solar cell according to the first embodiment aremeasured by X-ray photoelectron spectroscopy (XPS), a peak of coprousoxide exists in a range in which a binding energy value is 930 eV ormore and 934 eV or less. The peak of coprous oxide observed in a rangein which a binding energy value is 930 eV or more and 934 eV or less bymeasuring the photoelectric conversion layer 3 by XPS, is considered toinclude a Cu(0) peak considered to be derived from zerovalent copperconstituting metal copper, a Cu(I) peak considered to be derived frommonovalent copper constituting cuprous oxide, and a Cu(II) peak derivedfrom divalent copper constituting copper oxide (cupric oxide).Therefore, in a case where the photoelectric conversion layer 3 havingthe small number of heterogeneous phases is measured by XPS, when noother peak appears and the Cu(I) peak derived from copper suboxide(cuprous oxide) is a single peak, an almost symmetrical form about thepeak top is obtained.

FIG. 2 is a schematic diagram of a method for measuring thephotoelectric conversion layer 3 using X-ray photoelectron spectroscopy.Description will be given with reference to FIG. 2. First, when thephotoelectric conversion layer 3 included in the solar cell according tothe first embodiment is analyzed by XPS, in a peak of coprous oxideobserved in a range in which a binding energy value is 930 eV or moreand 934 eV or less, a point having the largest value, that is, a valueof a peak top 6 of coprous oxide is examined. A first intersection 8 anda second intersection 9 where a horizontal line 7 passing through avalue of ⅔ of the value of the peak top 6 intersects with the peak ofcoprous oxide are examined. Furthermore, a third intersection 11 where aperpendicular line 10 extending from the peak top 6 to the horizontalline 7 intersects with the horizontal line 7 is examined. Thereafter, adifference between a first length 12 formed by the first intersection 8and the third intersection 11 and a second length 13 formed by thesecond intersection 9 and the third intersection 11 is examined. Forconvenience, the first length 12 is referred to as L1, and the secondlength 13 is referred to as L2. This measurement is performed at ameasurement point set in a similar manner to measurement of thetransmittance. Specifically, a solar cell is divided into four equalparts in each of longitudinal and lateral directions. Next, by excludinga scribed portion, five points in a portion where the photoelectricconversion layer 3 exists are selected as measurement points, andmeasurement is performed by XPS at each of the points. At each of thesefive measurement points, a ratio of a difference between the firstlength (L1) and the second length (L2) (an absolute value of adifference of L1−L2/a longer value of L1 and L2) is determined. Forexample, when L1 is longer than L2, L1 is the longer value used as adenominator of the ratio. The absolute value of a difference of L1−L2 is|L1−L2| used as a numerator of the ratio. By averaging respective ratiosof the difference, an average of the ratios of the difference can bedetermined. These measurement points are selected from a range of 5% orless from intersections of lines obtained by equally dividing the solarcell into four equal parts in each of longitudinal and lateraldirections with respect to the longitudinal and lateral lengths of thesolar cell to be measured. Note that a measurement range is set within anarrower range than a scribe interval. The smaller number ofheterogeneous phases makes the difference between the first length andthe second length smaller.

Therefore, a case where the average of the ratios of the differencebetween the first length 12 formed by the first intersection 8 and thethird intersection 11 and the second length 13 formed by the secondintersection 9 and the third intersection 11 is 15% or less can meanthat the number of heterogeneous phases included in the photoelectricconversion layer 3 is sufficiently small. Therefore, it is possible toprevent a heterogeneous phase from serving as a center of recombinationof photoexcited carriers to reduce a conversion efficiency, to preventdeterioration of the quality of the photoelectric conversion layer 3itself, and to improve the efficiency of a solar cell. Furthermore, acase where the average of the ratios of the difference between the firstlength 12 formed by the first intersection 8 and the third intersection11 and the second length 13 formed by the second intersection 9 and thethird intersection 11 is 10% or less indicates that the number ofheterogeneous phases included in the photoelectric conversion layer 3 issmaller, and therefore can further improve the efficiency of the solarcell.

Incidentally, as described above, when the photoelectric conversionlayer 3 is measured by XPS, in order to make it possible to analyze astate in the photoelectric conversion layer 3, measurement is preferablyperformed after an n-type layer on the photoelectric conversion layer 3and the second electrode 5 thereon described later are removed by Ar ionetching or the like.

(n-Type Layer)

The n-type layer 4 is a layer existing between the photoelectricconversion layer 3 and the second electrode 5. In FIG. 1, the n-typelayer 4 is in direct contact with the photoelectric conversion layer 3and the second electrode 5.

It is desirable that the n-type layer 4 to be formed on thephotoelectric conversion layer 3 is free from excessive oxygen duringformation thereof. This is because, due to existence of oxygen in then-type layer 4, oxygen contained in the n-type layer 4 may react withcoprous oxide (contained in the photoelectric conversion layer 3) at aninterface between the photoelectric conversion layer 3 and the n-typelayer 4 to generate a heterogeneous phase such as copper oxide. Thisheterogeneous phase is more likely to be generated as the n-type layer 4contains more oxygen. Therefore, it is desirable that excessive oxygendoes not exist in the n-type layer 4 during formation thereof. Then-type layer 4 may include a plurality of layers including a bufferlayer and the like.

In formation of the n-type layer 4, for example, the n-type layer 4 canbe formed by co-sputtering sputtering targets of ZnO and GeO₂ in an Argas flow.

The n-type layer 4 preferably has a thickness of 5 nm or more and 100 nmor less. If the thickness of the n-type layer 4 is 5 nm or less, leakagecurrent may be generated when the n-type layer 4 has poor coverage todeteriorate characteristics. If the thickness of the n-type layer 4exceeds 100 nm, characteristics may be deteriorated due to an excessiveincrease in resistance of the n-type layer 4, or short-circuit currentmay decrease due to a decrease in transmittance. Therefore, thethickness of the n-type layer 4 is more preferably 10 nm or more and 50nm or less. In order to achieve a film with good coverage, the n-typelayer 4 preferably has a surface roughness of 5 nm or less. When thequality of the n-type layer is high, a solar cell which operates evenwith a film thickness of about 200 nm can be formed.

A conduction band offset (ΔE=Ecp−Ecn) which is a difference between theposition (Ecp (eV)) of a conduction band minimum (CBM) of thephotoelectric conversion layer 3 and the position (Ecn (eV)) of aconduction band minimum of the n-type layer 4 is preferably −0.2 eV ormore and 0.6 eV or less (−0.2 eV≤ΔE≤+0.6 eV). If the conduction bandoffset is larger than 0, the conduction band at a pn junction interfaceis discontinuous, and a spike is generated. If the conduction bandoffset is smaller than 0, the conduction band at the pn junctioninterface is discontinuous, and a cliff is generated. Both the spike andthe cliff are barriers to photogenerated electrons, and are preferablysmaller. Therefore, the conduction band offset is more preferably 0.0 eVor more and 0.4 eV or less (0.0 eV≤ΔE≤+0.4 eV). However, this is not thecase when conduction is performed using an in-gap level. The position ofCBM can be estimated using the following method. A valence band maximum(VBM) is actually measured by photoelectron spectroscopy which is anevaluation method of an electron occupancy level. Subsequently, CBM iscalculated by assuming a known band gap. However, at an actual pnjunction interface, an ideal interface is not maintained due to mutualdiffusion or generation of cation vacancy, and there is a highpossibility that the band gap may change. Therefore, CBM is alsopreferably evaluated by inverse photoelectron spectrometry directlyusing a reverse process of photoelectron emission. Specifically, anelectronic state of the pn junction interface can be evaluated byrepeating low energy ion etching and photoelectron/inverse photoelectronspectrometry on a surface of the solar cell.

(Second Electrode)

In FIG. 1, the second electrode 5 is in direct contact with the n-typelayer 4. As the second electrode 5, a transparent conductive film ispreferable. A similar material to the first electrode 2 is preferablyused for the transparent conductive film.

The composition and the like of the solar cell 100 are determined byX-ray photoelectron spectroscopy (XPS) and secondary ion massspectrometry (SIMS). The thickness and particle size of each layer aredetermined by performing observation of a cross section of the solarcell 100 with a transmission electron microscope (TEM) at 100,000 times.The surface roughness is determined by performing observation with anatomic force microscope (AFM).

(Third Electrode)

A third electrode according to the embodiment is an electrode of thesolar cell 100 and is a metal film formed on the second electrode 5 onthe opposite side to the photoelectric conversion layer 3. As the thirdelectrode, a conductive metal film such as Ni or Al can be used. Thethird electrode has a film thickness, for example, of 200 nm or more and2000 nm or less. In a case where a resistance value of the secondelectrode 5 is low and a series resistance component is negligible, thethird electrode may be omitted.

(Antireflection Film)

An antireflection film according to the embodiment is a film for easilyintroducing light into the photoelectric conversion layer 3. The secondelectrode 5 and the third electrode have two sides respectively. Oneside thereof opposes to the photoelectric conversion layer 3, and theother side thereof opposes to the antireflection film. Theantireflection film is formed on the second electrode 5 or the thirdelectrode on the opposite side to the photoelectric conversion layer 3.As the antireflection film, for example, it is desirable to use MgF₂ orSiO₂. Incidentally, in the embodiment, the antireflection film can beomitted. It is necessary to adjust the film thickness according to therefractive index of each layer, but it is preferable to performdeposition at about 70 to 130 nm (preferably 80 to 120 nm).

Here, a method for manufacturing the solar cell according to the presentembodiment will be described.

(Manufacturing Method)

FIG. 3 shows a flowchart of the method for manufacturing the solar cellaccording to the present embodiment.

A material to be the first electrode 2 is formed on the substrate 1 bysputtering or the like (S1). Next, the formed film is introduced into avacuum apparatus, and evacuation is performed (S2). A material to be thephotoelectric conversion layer 3 is formed under vacuum conditions bysputtering or the like (S3). After the photoelectric conversion layer 3is formed, the n-type layer 4 is formed (S4). Thereafter, a material tobe the second electrode 5 is formed by sputtering or the like (S5). Whenthe second electrode 5 is formed, the second electrode 5 may be a superstraight type or a substrate type.

The method for manufacturing the n-type layer 4 is not limited to theabove method. Examples of the method include chemical bath deposition(CBD), chemical vapor deposition (CVD), atomic layer deposition (ALD), acoating method, and an electrodeposition method.

The solar cell according to the first embodiment is a solar cellincluding a first electrode, a second electrode, and a photoelectricconversion layer disposed between the first electrode and the secondelectrode. When a transmittance of a plurality of parts of the solarcell is measured in a wavelength range of 700 to 1000 nm, an average ofthe transmittance of each of the parts of the solar cell is 60% or more.Therefore, it is possible to prevent a heterogeneous phase from servingas a recombination center of photoexcited carriers to reduce aconversion efficiency, to prevent deterioration of the quality of thephotoelectric conversion layer itself, and to form a solar cell having ahigh efficiency.

Second Embodiment

A second embodiment relates to a multi-junction solar cell. FIG. 4 showsa conceptual cross-sectional view of a multi-junction solar cell 200according to the second embodiment. The multi-junction solar cell 200shown in FIG. 4 includes the solar cell (first solar cell) 100 accordingto the first embodiment on a light incident side and a second solar cell201. A photoelectric conversion layer 3 of the second solar cell 201 hasa smaller band gap than the photoelectric conversion layer 3 of thesolar cell 100 according to the first embodiment. Note that themulti-junction solar cell according to the second embodiment alsoincludes a solar cell in which three or more solar cells are bonded.

The band gap of the photoelectric conversion layer 3 of the solar cell100 according to the first embodiment is about 2.0 eV. Therefore, theband gap of the photoelectric conversion layer 3 of the second solarcell 201 is preferably 1.0 eV or more and 1.4 eV or less. Thephotoelectric conversion layer 3 of the second solar cell 201 ispreferably a compound semiconductor layer including one or more of aCIGS-based compound with a high In content ratio, a CIT-based compoundwith a high In content ratio, a CdTe-based compound, and a CuO-basedcompound or crystalline silicon.

By using the solar cell according to the first embodiment as the firstsolar cell, it is possible to prevent reduction in conversion efficiencyof a bottom cell (second solar cell) due to absorption of light in anunintended wavelength range in the first solar cell. Therefore, it ispossible to form a multi-junction solar cell having a high efficiency.

Third Embodiment

A third embodiment relates to a solar cell module. FIG. 5 shows aconceptual perspective view of a solar cell module 300 according to thethird embodiment. The solar cell module 300 in FIG. 5 is a solar cellmodule in which a first solar cell module 301 and a second solar cellmodule 302 are laminated. The first solar cell module 301 is on a lightincident side, and the solar cell 100 according to the first embodimentis used therefor. For the second solar cell module 302, the second solarcell 201 is preferably used.

FIG. 6 shows a conceptual cross-sectional view of the solar cell module300. In FIG. 6, the structure of the first solar cell module 301 isshown in detail, and the structure of the second solar cell module 302is not shown. In the second solar cell module 302, the structure of thesolar cell module is appropriately selected according to thephotoelectric conversion layer 3 or the like of a solar cell used. Thesolar cell module in FIG. 6 includes a plurality of submodules 303 inwhich a plurality of the solar cells 100 is arranged in a lateraldirection and electrically connected in series, surrounded by a brokenline, and the plurality of submodules 303 is electrically connected inparallel or in series.

The solar cells 100 are scribed, and in the adjacent solar cells 100, asecond electrode 5 on an upper side of one solar cell is connected to afirst electrode 2 on a lower side of another solar cell (adjacent to theone solar cell). Like the solar cell 100 according to the firstembodiment, the solar cell 100 according to the third embodiment alsoincludes a substrate 1, the first electrode 2, a photoelectricconversion layer 3, an n-type layer 4, and the second electrode 5.

If output voltage varies depending on each module, current may flow backto a low voltage portion, or excess heat may be generated, and thereforethis leads to a decrease in output of the module.

By using the solar cell of the first embodiment, a solar cell suitablefor each wavelength band can be used. Therefore, it is possible togenerate power more efficiently than in a case of using a single unit,and the output of the whole module increases. Therefore, this isdesirable.

If the conversion efficiency of the whole module is high, an energyratio at which irradiation light energy becomes heat can be reduced.Therefore, it is possible to suppress reduction in efficiency due torise in temperature of the whole module.

Fourth Embodiment

A fourth embodiment relates to a solar power generation system. Thesolar cell module 300 according to the third embodiment can be used as apower generator for generating power in the solar power generationsystem according to the fourth embodiment. The solar power generationsystem according to the fourth embodiment generates power using a solarcell module, and specifically includes a solar cell module forgenerating power, a unit for converting generated electricity intopower, a storage unit for storing generated electricity, and a load forconsuming generated electricity. FIG. 7 shows a conceptual diagram ofthe configuration of a solar power generation system 400 according tothe fourth embodiment. The solar power generation system in FIG. 7includes a solar cell module 401 (300), a converter 402, a storagebattery 403, and a load 404. Either one of the storage battery 403 andthe load 404 may be omitted. The load 404 may have a configurationcapable of utilizing electric energy stored in the storage battery 403.The converter 402 is an apparatus including a circuit or an element forperforming voltage transformation or power conversion such as a DC-DCconverter, a DC-AC converter, or an AC-AC converter. For theconfiguration of the converter 402, it is only required to adopt asuitable configuration according to power generation voltage, and theconfigurations of the storage battery 403 and the load 404.

A solar cell included in the submodule 303 (included in the solar cellmodule 300) which has received light generates power, and electricenergy thereof is converted by the converter 402 and stored in thestorage battery 403 or consumed by the load 404. To the solar cellmodule 401, a sunlight tracking drive apparatus for constantly directingthe solar cell module 401 to the sun, a light collector for collectingsunlight, an apparatus for improving a power generation efficiency, andthe like are preferably added.

The solar power generation system 400 is preferably used for real estatesuch as a residence, commercial facilities, and a factory, or used formovable items such as a vehicle, an aircraft, and an electronic device.An increase in the amount of power generation is expected by using thesolar cell having excellent conversion efficiency according to the firstembodiment for the solar cell module 401.

Hereinafter, the present invention will be described more specificallybased on Embodiments, but the present invention is not limited to thefollowing Embodiments.

EMBODIMENTS

A top cell is manufactured, and a light transmittance, an XPS value, anda conversion efficiency are measured.

Embodiment 1

On a white sheet glass substrate, an ITO transparent conductive film asa first electrode on a back surface side is deposited, and a SnO₂transparent conductive film doped with Sb is deposited thereon. On thetransparent first electrode, a film of a coprous oxide compound isformed by heating at 450° C. by a sputtering method in an atmospherehaving a ratio between oxygen and oxygen+argon gas (O₂/(Ar+O₂) ratio) of0.078. Thereafter, n-type layer Zn_(0.8)Ge_(0.2)O_(x) is deposited onthe p-coprous oxide layer (photoelectric conversion layer) by asputtering method at room temperature, and MgF₂ is deposited thereon asan antireflection film. Thereafter, an AZO transparent conductive filmis deposited as a second electrode on a front surface side. At the timeof depositing the second electrode on the front surface side, it isnecessary to form a film at room temperature in order to suppressoxidation of coprous oxide. For example, by using AZO, a film with lowresistance can be obtained even at room temperature. A target of AZOpreferably has a ratio of Al₂O₃ of about 2 wt % to 3 wt % with respectto ZnO, but is not limited thereto as long as having a sufficiently lowresistance value and a high transmittance with respect to an element.

An average transmittance is measured as follows.

As an apparatus, an ultraviolet-visible near-infrared spectrophotometer(model: UV-3101PC manufactured by Shimadzu Corporation) was used. Asolar cell is divided into four equal parts in each of longitudinal andlateral directions, and five points are selected from intersections oflines obtained by dividing the solar cell into four equal parts in eachof longitudinal and lateral directions. A measurement point was selectedfrom a range of 5% or less from each of the intersections with respectto the longitudinal and lateral lengths of the solar cell to bemeasured. The transmittance at each of the measurement points isobtained by determining a transmittance at every 1 nm in a range of 700to 1000 nm and by averaging the determined transmittances. An averagetransmittance of the solar cell was determined from the transmittances(measured in this way) at the five measurement points, and is shown inTable 1.

An XPS value was measured as follows. In elemental analysis of aphotoelectric conversion layer by XPS, first, a second electrode and ann-type layer were removed from a solar cell using Ar ion etching toexpose a p-type photoelectric conversion layer. Next, for the exposedphotoelectric conversion layer, the XPS value was measured using thefollowing apparatus and measurement conditions. As a device used, AXISUltra DLD manufactured by Shimadzu Corporation was used. As anexcitation source, monochro (Al—Kα) (15 kV×15 mA) was used. As ameasurement mode, spectrum was used for an analyser mode, and hybrid wasused for a lens mode. A photoelectron extraction angle was set to 45°. Acapture region was set to a range in which a binding energy value was 0to 1200 eV for wide scan, and was set to a range of 926 to 942 eV ofCu2p and a range of 278 to 294 eV of C1s in which a main peak of coprousoxide can be observed for narrow scan. At this time, pass energy was setto 160 eV for wide scan and was set to 10 eV for narrow scan.Measurement was performed such that a binding energy value was increasedby every 0.1 eV. Charge correction was performed by setting a C1s peakof surface contaminated hydrocarbon to 284.8 eV.

This measurement was performed at five points on the photoelectricconversion layer, and an average of ratios of the difference thereof wasdetermined. In Table 1, a case where an average range of the ratios ofthe difference was within 10% was evaluated to be ⊙, a case where anaverage range of the ratios of the difference was within 15% wasevaluated to be ◯, and a case where an average range of the ratios ofthe difference was larger than 15% was evaluated to be X.

A method for measuring a conversion efficiency is as follows.

Using a solar simulator simulating a light source of AM 1.5 G, the lightquantity is adjusted such that the light quantity becomes 1 sun using aSi cell as a reference under the light source. The temperature is 25° C.When the horizontal axis indicates voltage and the vertical axisindicates current density, a point intersecting with the horizontal axisis Voc. Voltage sweep is performed from a value covering Voc with avoltmeter (for example, 1.6 V) up to a range in which short circuitcurrent density (Jsc) can be measured (minus region, for example, −0.4V), and a current value is measured at this time. A value obtained bydividing the current value by the area of the solar cell is a currentdensity (mA/cm²), and a value of a current density at an applied voltageof 0 V is Jsc.

Efficiency (η) is calculated as follows.

η=Voc×Jsc×FF/P×100

P is incident power density by calibrating the simulated sunlight of AM1.5 G with the reference solar cell.

FF is calculated as follows.

FF=Vmpp×Jmpp/(Voc×Jsc).

Vmpp and Jmpp are values of V and J at the point where the product ofV×J is maximum.

These results are shown in Table 1. Tables 2 and 3 show results ofEmbodiments 7 to 14 and Comparative Embodiments 4 to 7. Tables 1 to 3show solar cell efficiencies (FFs) in other Embodiment and ComparativeExamples calculated by using the value in Embodiment 1 as a reference.

Embodiments 2 to 6

Manufacture and measurement were performed in a similar manner toEmbodiment 1 except that the ratio between oxygen and argon gas was setas shown in Table 1.

Comparative Example 1

Manufacture and measurement were performed in a similar manner toEmbodiment 1 except that an oxygen gas flow existed and Zn+Ge was usedas the target in depositing the n-type layer.

Comparative Example 2

Manufacture and measurement were performed in a similar manner toComparative Example 1 except that ZnO+Ge was used as the target indepositing the n-type layer.

Comparative Example 3

Manufacture and measurement were performed in a similar manner toComparative Example 1 except that Zn+GeO₂ was used as the target indepositing the n-type layer.

Embodiments 7 to 10, Comparative Examples 4 and 5

Manufacture and measurement were performed in a similar manner toEmbodiment 1 except that tungsten (W) was used as the first electrodeand the ratio between oxygen and argon gas was set as shown in Table 2.

Embodiment 11

Manufacture and measurement were performed in a similar manner toEmbodiment 8 except that a film of coprous oxide was formed at 400° C.

Embodiment 12

Manufacture and measurement were performed in a similar manner toEmbodiment 9 except that a film of the coprous oxide compound was formedat 600° C.

Embodiment 13

Manufacture and measurement were performed in a similar manner toEmbodiment 1 except that a film of the coprous oxide compound was formedat 400° C.

Embodiment 14

Manufacture and measurement were performed in a similar manner toEmbodiment 1 except that a film of the coprous oxide compound was formedat 600° C.

Comparative Example 6

Manufacture and measurement were performed in a similar manner toEmbodiment 1 except that a film of the coprous oxide compound was formedat 350° C.

Comparative Example 7

Manufacture and measurement were performed in a similar manner toEmbodiment 1 except that a film of the coprous oxide compound was formedat 750° C.

TABLE 1 Average Ratio of transmittance Efficiency O₂/(Ar + O₂) (%) XPSratio Example 1 0.078 75 ⊙ 1 Example 2 0.076 72 ⊙ 0.995 Example 3 0.08072 ⊙ 0.994 Example 4 0.082 63 ◯ 0.88 Example 5 0.074 61 ◯ 0.84 Example 60.084 61 ◯ 0.82 Comparative 0.078 43 X 0.02 Example 1 Comparative 0.07849 X 0.1 Example 2 Comparative 0.078 56 X 0.14 Example 3

TABLE 2 Average Ratio of reflectance Efficiency O₂/(Ar + O₂) (%) XPSratio Example 7 0.080 58 ⊙ 0.91 Example 8 0.078 57 ◯ 0.89 Example 90.082 56 ◯ 0.8 Example 10 0.076 49 ◯ 0.79 Comparative 0.074 17 ◯ 0.71Example 4 Comparative 0.084 16 ◯ 0.71 Example 5 Example 11 0.078 54 ◯0.81 Example 12 0.082 51 ◯ 0.84

TABLE 3 Average Ratio of transmittance Efficiency O₂/(Ar + O₂) (%) XPSratio Example 1 0.078 75 ⊙ 1 Example 13 0.078 74 ⊙ 0.995 Example 140.078 61 ◯ 0.81 Comparative 0.078 10 X X Example 6 Comparative 0.078 3 XX Example 7

Tables 1 to 3 indicate that, when Embodiments 1 to 6 are compared withComparative Embodiments 1 to 3, an excellent efficiency can be achievedby a fact that an average transmittance is 60% or more when thetransmittance of a solar cell is measured in a wavelength range of 700to 1000 nm. Furthermore, a better efficiency can be achieved with anaverage transmittance of 70%.

When Embodiments 7 to 14 are compared with Comparative Examples 4 and 5,it is found that an excellent efficiency can be achieved even by a factthat an average reflectance is 49% or more when the reflectance of asolar cell is measured in a wavelength range of 700 to 1000 nm.

In solar cells manufactured at the same temperature, the efficiencyratios in Embodiments 1 to 6 using transparent electrodes were higherthan those in Embodiments 7 to 10 using metal electrodes. This isbecause existence of an oxide on at least a part of the first electrodecan moderately suppress the reaction between oxygen and the electrode atthe time of forming coprous oxide.

In addition, a conversion efficiency of a solar cell can be improvedalso by a fact that, when a photoelectric conversion layer is analyzedby XPS, in a peak of coprous oxide observed in a range in which abinding energy value is 930 eV or more and 934 eV or less, there are afirst intersection and a second intersection where a horizontal linepassing through a value of ⅔ of a peak top of coprous oxide intersectswith the peak of coprous oxide, there is a third intersection where aperpendicular line extending from the peak top to the horizontal lineintersects with the horizontal line, and an average of ratios of adifference between a first length formed by the first intersection andthe third intersection and a second length formed by the secondintersection and the third intersection is 15% or less. This is because,due to the small number of heterogeneous phases existing in thephotoelectric conversion layer, it is possible to prevent aheterogeneous phase from serving as a center of recombination ofphotoexcited carriers to reduce a conversion efficiency, to preventabsorption of light in an unintended wavelength range, and to improvethe quality of the photoelectric conversion layer. Therefore, a shortcircuit current density in the photoelectric conversion layer (p layer,coprous oxide) can be improved. In addition, by forming a favorableinterface, a defect level of the interface is reduced, and an opencircuit voltage can be increased. Furthermore, by suppressing a leakcomponent and forming a good contact, a fill factor is also improved,and the efficiency can be improved.

While certain embodiments have been described, these embodiments havebeen presented by way of examples only, and are not intended to limitthe scope of the inventions. Indeed, the novel embodiments describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A solar cell comprising: a first electrode; asecond electrode; and a photoelectric conversion layer disposed betweenthe first electrode and the second electrode, wherein, when atransmittance of the solar cell is measured in a wavelength range of 700to 1000 nm, an average of the transmittance of the solar cell is 60% ormore.
 2. The solar cell according to claim 1, wherein the average of thetransmittance is 70% or more.
 3. The solar cell according to claim 1,wherein the photoelectric conversion layer contains coprous oxide. 4.The solar cell according to claim 3, wherein, when the photoelectricconversion layer is analyzed by X-ray photoelectron spectroscopy, in apeak of the coprous oxide observed in a range in which a binding energyvalue is 930 eV or more and 934 eV or less, there are a firstintersection and a second intersection where a horizontal line passingthrough a value of ⅔ of a peak top of the coprous oxide intersects withthe peak of the coprous oxide, there is a third intersection where aperpendicular line extending from the peak top to the horizontal lineintersects with the horizontal line, and an average of ratios of adifference between a first length formed by the first intersection andthe third intersection and a second length formed by the secondintersection and the third intersection is 15% or less.
 5. The solarcell according to claim 4, wherein the average of ratios of thedifference is 10% or less.
 6. A multi-junction solar cell using thesolar cell according to claim
 1. 7. A solar cell module using the solarcell according to claim
 1. 8. A solar cell module using themulti-junction solar cell according to claim
 6. 9. A solar powergeneration system for performing solar power generation using the solarcell module according to claim
 7. 10. A solar power generation systemfor performing solar power generation using the solar cell moduleaccording to claim
 8. 11. A solar cell comprising: a first electrode; asecond electrode; and a photoelectric conversion layer disposed betweenthe first electrode and the second electrode, wherein, when areflectance of the solar cell is measured in a wavelength range of 700to 1000 nm, an average of the reflectance of the solar cell is 49% ormore.
 12. The solar cell according to claim 11, wherein the average ofthe reflectance is 58% or more.
 13. The solar cell according to claim11, wherein the photoelectric conversion layer contains coprous oxide.14. The solar cell according to claim 13, wherein, when thephotoelectric conversion layer is analyzed by X-ray photoelectronspectroscopy, in a peak of the coprous oxide observed in a range inwhich a binding energy value is 930 eV or more and 934 eV or less, thereare a first intersection and a second intersection where a horizontalline passing through a value of ⅔ of a peak top of the coprous oxideintersects with the peak of the coprous oxide, there is a thirdintersection where a perpendicular line extending from the peak top tothe horizontal line intersects with the horizontal line, and an averageof ratios of a difference between a first length formed by the firstintersection and the third intersection and a second length formed bythe second intersection and the third intersection is 15% or less. 15.The solar cell according to claim 14, wherein the average of ratios ofthe difference is 10% or less.
 16. A multi-junction solar cell using thesolar cell according to claim
 11. 17. A solar cell module using thesolar cell according to claim
 11. 18. A solar cell module using themulti-junction solar cell according to claim
 16. 19. A solar powergeneration system for performing solar power generation using the solarcell module according to claim
 17. 20. A solar power generation systemfor performing solar power generation using the solar cell moduleaccording to claim 18.